(1) Field of the Invention
The present invention relates to a storage device using a ferroelectric material, in particular to a technique for improving reliability of the storage device at a time of rewriting data.
(2) Description of the Related Art
In recent years, ferroelectric memories (FeRAM: ferroelectric random access memory) using a ferroelectric material have been developed actively. The ferroelectric memories are characterized by having nonvolatility and being rewritable at high-speed. However, the ferroelectric memories can be rewritten only a limited number of times, because fatigue and deterioration of their ferroelectric films gradually progress as the memories are rewritten repeatedly.
The fatigue and the deterioration are caused by an inversion of a polarization. To suppress the fatigue and the deterioration, an asymmetric programming method has been developed, in which data “1” is represented by a positive saturation polarization, and data “0” is represented by a polarization that is approximate to a zero polarization (e.g. Japanese laid-open patent publication No. 2001-308291). This method reduces the change in a polarization level, and thereby suppresses the fatigue and the deterioration of the ferroelectric film. Accordingly, this increases the limit on the number of rewritings.
For realizing higher rewriting speed in such ferroelectric memories using the asymmetric programming method, it is necessary to shorten the time required for rewriting each piece of data.
However, if the time for the rewriting is shortened, a rewriting failure might occur. This decreases the reliability of the ferroelectric memories.